1. Field of the Invention
The present invention relates to an improvement on a multipoint distance measuring device arranged to compute distance measuring information for a plurality of distance measuring points and a camera having the multipoint distance measuring device.
2. Description of Related Art
Some of known cameras are arranged to permit the user to vary the focal length of a photo-taking optical system of the camera from one focal length over to another as desired according to the composition of a picture of an object of shooting. Meanwhile, it is known that an active-type distance measuring device is arranged to project a light flux onto a distance measuring object from a light emitting element and to measure a distance to the object by receiving a reflection light flux reflected by the object as a result of the light projection.
It is also known that some cameras are provided with a multipoint distance measuring device. The multipoint distance measuring device is arranged to permit taking a picture of a desired object in an in-focus state in whatever position the object may be located on a photo-taking image plane, by setting a plurality of distance measuring points on the photo-taking image plane in the horizontal direction of the photo-taking image plane.
FIG. 8 shows, by way of example, a multipoint distance measuring device adapted for an automatic-focusing type camera. Referring to FIG. 8, a light emitting element array 1 is composed of a plurality of light emitting elements R2, R1, C, L1 and L2. A light projecting lens 2 is arranged to project light fluxes generated by the light emitting elements R2 to L2 respectively toward distance measuring targets r2, r1, c, l1 and l2 which are arranged side by side in the horizontal direction of a photo-taking image plane. A light receiving element 3 which is, for example, a PSD (a semiconductor position detector) is arranged to output a signal which varies according to the incident position of a light flux incident thereon. A light receiving lens 4 is arranged to cause a light flux reflected by an object located in each of the distance measuring targets as a result of the light projection to be led to the light receiving element 3. In the case of this example, the light emitting elements R2 to L2 are time-serially lighted up one after another. Information on a distance to each of the distance measuring targets r2 to l2 is obtained from the light receiving element 3 with the light emitting elements R2 to L2 lighted up. With the multipoint distance measuring device arranged in this manner, an automatic focusing action can be appositely carried out irrespective of the position of the object within the photo-taking image plane.
However, if such a camera is made to have the focal length of its photo-taking optical system arranged to be variable, a problem arises as follows.
In the variable-focal-length camera (hereinafter will be referred to as a zoom camera), the photo-taking angle of view varies with the change-over of focal length of the photo-taking optical system. When the zoom camera is arranged to perform an automatic focusing action in the above-stated manner, therefore, an object located outside the photo-taking image plane might be included in the distance measuring objects. As shown in FIG. 8, a photo-taking angle of view xcex2 on the side of longer focal length of the photo-taking optical system is narrower than the photo-taking angle of view xcex1 on the side of shorter focal length. Therefore, if a plurality of distance measuring targets r2 to l2 are set in such a manner as to correspond to the photo-taking angle of view xcex1, objects located outside the photo-taking angle of view xcex2, i.e., objects within the distance-measuring targets r2 and l2, come to be included in the distance measuring objects at a longer focal length. Under such a condition, the result of distance measurement might be caused to be erroneous by inclusion of information on the unnecessary distance measurement. Besides, in such a case, electric energy is wasted by unnecessarily lighting up the light emitting elements R2 and L2.
To solve this problem, a zoom camera disclosed in Japanese Patent Publication No. Hei 3-80290 is arranged such that, when the focal length of the zoom camera is changed over to a longer focal length, a distance measuring action on a distance measuring target located outside the range of multipoint distance measurement is inhibited. In other words, in the case of FIG. 8, when the focal length of the photo-taking optical system corresponds to the angle of view xcex2, distance measuring actions with the light emitting elements R2 and L2 are inhibited.
The multipoint distance measuring device of the conventional zoom camera is arranged to minimize a period of time required for multipoint distance measurement over the wide range of view on the shorter focal length side and yet to be capable of adequately carrying out a distance measuring action on the longer focal length side.
Some of zoom cameras of these days are arranged to have a telephoto function. The camera of that kind must be capable of measuring distances up to a long distance.
However, on the contrary, cameras are required to be compact in size. This requirement causes reduction in size of the distance measuring optical system and also in battery capacity. As a result, it has become difficult to obtain a sufficient amount of signal, i.e., to obtain an adequate distance measuring capability.
In addition to the focal length, a shot noise due to external light (disturbance noise) is considered also to be one of factors affecting the distance measuring capability. Within an area where the distance measuring capability is affected by the shot noise, the distance measuring capability lowers accordingly as the amount of external light increases, with signal components assumed to be unvarying. In a case where the amount of external light is large, the distance measuring capability of the multipoint distance measuring device becomes insufficient at all distance measuring points, and it tends to become impossible to obtain sufficiently-reliable values of distance measurement.
This problem may be solved by increasing a distance measuring period of time per distance measuring point. Such a solution, however, causes a total period of time required in measuring distances at all points of the multipoint distance measurement to become too long in a case where there are many distance measuring points. Such a long distance measuring period of time tends to cause the user of the camera to miss a shutter-operating opportunity or to take a blurred picture due to image shaking.
In accordance with an aspect of the invention, there is provided an automatic focusing apparatus arranged to project light onto objects caught at a plurality of detecting areas in respective different directions, to receive, at a light receiving part, reflected light resulting from the light projection and to compute data for focusing on the basis of an output of the light receiving part resulting from the light projection on each object, the automatic focusing apparatus comprising a varying circuit which varies the number of the detecting areas, and a setting circuit which sets a maximum period of time required for computing the data, according to the number of the detecting areas varied by the varying circuit, in such a way as to make the length of distance measuring time and the distance measuring capability of the automatic focusing apparatus adequate.
In accordance with another aspect of the invention, there is provided an automatic focusing apparatus arranged to project light onto objects caught at a plurality of detecting areas in respective different directions, to receive, at a light receiving part, reflected light resulting from the light projection and to compute data for focusing on the basis of an output of the light receiving part resulting from the light projection on each object, the automatic focusing apparatus comprising a light projecting circuit which causes the light projection to be repeatedly made on the object located at each of the detecting areas, a varying circuit which varies the number of the detecting areas, and a setting circuit which sets a maximum number of times of the light projection caused by the light projecting circuit for each detecting area, according to the number of the detecting areas varied by the varying circuit, in such a way as to make the length of distance measuring time and the distance measuring capability of the automatic focusing apparatus adequate.
In accordance with a further aspect of the invention, there is provided an automatic focusing apparatus arranged to project light onto objects caught at a plurality of detecting areas in respective different directions, to receive, at a signal-storing-type light receiving part, reflected light resulting from the light projection and to compute data for focusing on the basis of an output of the light receiving part resulting from the light projection on each object, the automatic focusing apparatus comprising a varying circuit which varies the number of the detecting areas, and a setting circuit which sets a maximum period of time for storing a signal at the light receiving part, according to the number of the detecting areas varied by the varying circuit, in such a way as to make the length of distance measuring time and the distance measuring capability of the automatic focusing apparatus adequate.
These and other aspects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.